The present invention relates to semiconductor devices and more particularly, to improved methods of forming memory cell capacitor plates from materials that do not form insulators upon exposure to oxygen. Materials with such properties might include oxygen-resistant materials, conductive materials that form conductive oxides upon exposure to oxygen, or the conductive oxides themselves for use in memory cell capacitor devices.
Semiconductor manufacturers must continually improve the power and performance of semiconductor devices while keeping the device size to a minimum. In an effort to maintain a small device size in the manufacture of integrated circuit devices such as memory cells and memory cell capacitor structures, most semiconductor manufacturers reduce individual components of the memory device to minimal dimensions. To accomplish that purpose, manufacturers are turning to different material alternatives having the desired characteristics that would reduce the device area consumed by the components. However, new problems arise as traditional manufacturing processes are applied to these new materials. For example, ferroelectric materials offer advantageous characteristics such as high dielectric constants, remnant polarization, and low coercive voltages, which make the use of these materials increasingly attractive in the semiconductor industry.
An illustrative example can be found in DRAM applications, where the high dielectric constants of ferroelectric materials allow for increased capacitance per cell volume. This translates into a DRAM cell size reduction by as much as a factor of 20 in comparison with DRAM cells presently in use that employ silicon nitride or silicon oxide dielectrics. Moreover, the remnant polarization allows for the storage of states, much like the storage of states in a magnetic domain of, for example, a ferromagnetic material. This polarization characteristic of ferroelectric materials permits information to be stored indefinitely as in magnetic materials, without the need to provide an applied field or voltage to maintain that memory. This makes ferroelectric materials an excellent material choice for manufacturing nonvolatile memory arrays. Furthermore, the low coercive voltages exhibited by ferroelectric materials advantageously allow these nonvolatile memory arrays to switch states, among other modes of operation, using standard supply voltages, for example, between about 3 and about 5 volts.
Ferroelectric materials generally require high processing temperatures to achieve the desired crystal structure that will provide the preferred characteristics, and the formation of the ferroelectric phase depends on the availability of oxygen. Therefore, ferroelectric materials are generally deposited in an oxygen-containing environment, which might result in incompatibility between ferroelectric materials and the conductive materials used within the same device. By way of example, a capacitor structure might have a capacitor storage element made of ferroelectric materials and capacitor plates made of a conductive metal. The oxygen present in the deposition environment of ferroelectrics may very well form an oxide with the conductive metal used to form capacitor plates. And since most oxides have an insulating effect, this would drastically impact the capacitor properties in a detrimental manner by cutting off electrical contact at the interface where the oxide is formed. The oxygen atmosphere processing of the ferroelectric film places stringent materials requirements on the capacitor plates, which generally should be made of materials that are substantially nonreactive with oxygen. Materials that could be used compatibly with ferroelectrics might include oxygen-resistant materials, conductive materials that form conductive oxides, or the conductive oxides themselves for use in memory cell capacitor devices. Examples of these materials might include, but are not limited to, platinum, ruthenium, ruthenium oxide, iridium, and iridium oxide. However, these unconventional materials do not lend themselves well to traditional plasma etching techniques.
By way of example, platinum is a relatively chemically inert material that does not respond very well to etching. FIG. 1 depicts an etched platinum-containing layer 100 which illustrates the problems encountered in the etching of platinum. Platinum-containing layer 100 has been sputter etched to form the desired structure, often with a noble gas such as argon. The sputtering displaces some platinum ions, which tend not to land on the chamber walls, but instead, hit the sidewalls 102 and redeposit upon the platinum-containing layer 100. Upon removal of a photoresist mask previously disposed over platinum-containing layer 100, fang-like structures which are commonly termed as veils 104 are formed. These structures may also be referred to as redeposit, fences, crowns, and ears, among others. The prominence of veils 104 relates inversely with the tapering angle of platinum-containing layer 100, that is, the more vertical the profile, the more prominent the veils. However, excessive taper angles that would avoid veil formation would also result in lower packing densities, which would be inefficient and therefore not be feasible in these applications.
There are many disadvantages with this particular aspect of platinum etching. Veils 104, which protrude above the top surface of platinum-containing layer 100, do not allow for a flat surface upon which subsequent layers may be deposited. Moreover, the fang-like structure of veils 104 provide sharp points on the structure, that are areas that could possibly generate very high electric fields even with extremely small voltages, which has a very high potential for breaking down. The preferred configuration would probably be a mesa structure that would have steep profile angles, i.e., more vertical profiles, to allow a more compact packing density, yet would have no sharp protrusions that might cause device failure.
Therefore, there are desired improved methods that allow for the manufacture of capacitor plates made from materials that do not react with oxygen to form insulators while avoiding the aforementioned problems associated with etching these materials.